This invention relates to electric self-cleaning cooking ovens and more particularly to power control relay switching circuits for such ovens.
Electric self-cleaning ovens are typically provided with a broil heating element disposed proximate the top wall of the oven and a bake element disposed proximate the bottom wall of the oven. A typical relay switching circuit known in the art for controlling energization of these elements is illustrated in FIG. 1. In this circuit, relay contacts 1 and 2 switchably electrically connect one terminal of the bake element 3 and the broil element 4 respectively to L1. Relay 5 connects the other terminal of both heating elements to power source L2 via a thermal limit switch 6. Typically the bake element has a power rating which is roughly 75% of the broil element rating.
During normal operation the temperature in the oven is maintained within acceptable limits by cycling the relay switches. For example, energization of the broil element in the bake operating mode is typically cycled to operate at one-quarter power when the bake element is operated at full power, and in the broil mode, the bake element is switched to its open state and the broil element is operated at full power or duty cycled depending on the broil mode selected. Consequently under normal operating conditions, the maximum total power is applied in the broil mode and the oven is designed to keep the surface temperature of the oven cabinet within acceptable temperature limits under such conditions. However, an abnormal operating condition could arise in which the three relays fail closed. In this worst case condition both the bake and broil elements would be energized at full power simultaneously. Under such conditions the oven temperature may rise to a level which causes the cabinet surface temperature to exceed normal operating temperatures. The thermal limit switch 6 is mounted externally of the oven on the outer surface of the range cabinet to provide protection against such an occurrence. Switch 6 is operative to interrupt energization of the heating elements in the event the temperature of the oven cabinet proximate the switch exceeds its threshold temperature.
This arrangement works satisfactorily, however, the use of the limit switch and its associated wiring adds cost to the design. It would be desirable to provide a less costly circuit arrangement which protects against excessive temperature conditions in the event of worst case failures of the switching devices without adversely affecting heating performance.
It is therefore, a primary object of the present invention to provide an improved switching arrangement for use in self-cleaning ovens which provides reliable protection against excessive temperatures in the event of switching device failures and which uses fewer components and is less costly than arrangements known in the art.